I have an old fiberglass 8 1/2 foot South Bend model 3370 that I'm pairing with an equally old Heddon 320 reel for bluegill/bass fishing. Any thoughts on what modern line would be a good match? The rod is rated for a HDH line, which from what I can tell is a 6 wgt DT...any thoughts on Cortland's Sylk line?

While there were a few better quality South Bends, for the most part they're not a rod that's going to make you fall in love.
What you should look for is a line that is supple and not too costly.
A line like the 333 or step up to a Hook&Hackle High Floater or even a regular 444peach
Try the lines that you already have first to get an idea of what might work best. You may find that you have a suitable line already.
Some of those South Bends were pretty slow and I think that you may find that a 5wt feels better than a 6.